Packet data communication systems are known in both the land mobile and cellular environments. In the land mobile environment, the packet data system includes a packet router, a central controller, a plurality of base sites and a plurality of communication units (e.g., mobile radios, portable radios, or wireless data terminals). In the cellular environment, the packet data system includes a packet router, one or more base site controllers, a plurality of base sites, and a plurality of communication units (e.g., radiotelephones or wireless data terminals).
Land mobile packet data communication systems are typically wide coverage area systems (i.e., one base site services a large coverage area). Thus, a data packet transmission most often occurs from one base site to the communication unit. In a typical data packet transmission, the packet router provides a data packet from a land user to the central controller via one of several communication protocol layers. For example, a first protocol layer (layer 1) comprises a land protocol, such as T1 channel framing, in the fixed network. A second protocol layer (layer 2) typically contains error correction information to allow the communication unit to correct bit errors incurred during transmission of the packet from a base site. A third protocol layer (layer 3) typically includes packet routing information (e.g., the appropriate Internet protocol (IP) address for the communication unit). A fourth protocol layer (layer 4) typically includes an indication of the amount of packets containing the data in the packet data communication. Finally, upper protocol layers (layer 5 and above) contain the actual data in the packet. Thus, the higher the protocol layer used for transmission, the more information that must be analyzed by the communicating entity (i.e., base site, central controller, packet router) and, accordingly, the longer the communication time period. Therefore, protocol layers allow the data to be transferred efficiently from the data source (i.e., sender) to the data recipient (i.e., communication unit) by permitting the intermediate entities to process only the information necessary to deliver the packet to the recipient. The transmission from the packet router to the central controller is generally a layer 3 transmission.
The central controller provides the data packet via layer 1 to the base site serving the communication unit. The base site divides the data packet into data blocks and transmits the data blocks sequentially to the communication unit via layer 2. Thus, layer 2 transmission occurs between the base site and the communication unit only because the communication channel between the base site and the communication unit is the most susceptible to bit errors. Therefore, by limiting layer 2 transmission to communications between the base site and the communication unit, present wide area land mobile systems are throughput efficient. However, wide area land mobile systems are spectrally inefficient because each packet transmission occupies a communication resource, such as a time slot of a radio frequency carrier, and blocks other communication units in the wide coverage area (which may contain many communication units) from using the resource until the packet has been transmitted.
To overcome the spectral inefficiency of wide area systems, land mobile systems have been advantageously incorporating frequency reuse concepts to improve spectral efficiency, thereby creating cellular-type land mobile systems. Like cellular systems, cellular-type land mobile systems include multiple base sites having relatively small coverage areas. Thus, a communication unit is likely to be handed off from one base site to another during the transmission of a data packet as the communication unit moves throughout the communication system. However, packet data transmission techniques of wide area land mobile systems are not readily adaptable to packet data transmissions of cellular-type land mobile systems. With existing wide area land mobile techniques, the data blocks transmitted from the original serving base site after handoff would not be received by the communication unit and would be lost. Therefore, in order to provide the communication unit with the lost information, the communication unit must request the land user to resend the complete data packet so that the packet router can reroute the packet to the handoff base site that is now serving the communication unit. The supplemental transmission of the data packet from the land user inherently increases the overall transmission delay of the data packet and, in addition, is subject to the transmission control protocol (TCP) flow control mechanism in the land system. The TCP flow control mechanism produces additional delays and reduced throughput for packet transmissions from land users in order to control land system congestion. Thus, by requiring the land user to resend the packet upon handoff of the communication unit, unnecessary (and possibly unacceptable) delays would be incurred in a cellular-type system utilizing wide area packet data transmission techniques.
To obviate TCP flow control mechanism delays and throughput reduction in cellular systems, cellular digital packet data (CDPD) systems provide layer 2 transmission between the base site controller (which typically controls multiple base sites) and the communication unit. Thus, when a communication unit is handed off in a cellular system, the communication session is not interrupted because the session is between the base site controller and the communication unit, as opposed to between the base site and the communication unit as in wide area land mobile systems. Therefore, upon handoff, the base site controller provides the unreceived portion of the data packet to the handoff base site without requesting a re-transmission from the land user. However, requiring layer 2 evaluation of the low BER link between the base site and the base site controller and processing of the data blocks at the base site controller increases the effective transmission delay of the data packet and, accordingly, reduces data throughput efficiency.
Therefore, a need exists for a method and apparatus for providing packet data to a communication unit in a packet data communication system that provides the throughput efficiency of wide area land mobile systems and the spectral efficiency of cellular systems.